Production of Biocrude From Biomass by Acidic Subcritical-Water Followed by Alkaline Supercritical-Water Two-Step Liquefaction

Increased demand in transportation fuels, environmental concerns and depletion of fossil fuel require development of efficient conversion technologies for second-generation biofuels.  The main objective of this work is to efficiently liquefy biomass into energy-dense biocrude. A novel two-step process is proposed in which acidic subcritical-water followed by alkaline supercritical-water media are utilized for the liquefaction.  The concept is tested with switchgrass. The first step is carried out at 200 °C in acidic subcritical water to liquefy hemicelluloses to biocrude while avoiding the repolymerization reactions which would otherwise produce char. In the second step, the remaining un-liquefied biomass (biomass-H) is subjected to supercritical water at 380 °C with Ca(OH)₂ as catalyst for minimizing the formation of char, enhancing lignin solubilization and therefore increasing liquefaction of the remaining polysaccharides toward biocrude. The proposed two-step liquefaction produces significantly higher amount of biocrude as compared to the traditional one-step process.  The yield of biocrude from the proposed process is 40% on mass basis and 67% on energy basis of the feedstock biomass.